A. 8. Lucas — The Headon Beds of the Isle of Wight. 101 



conspicuous, well-developed, and easily recognized member of the 

 fresh-water series was brought home to us in the field. 



The next beds to compare are those between these admitted 

 Osborne marls and the brackish-marine series of Colwell Bay, and 

 those between the similar mottled marls and a similar brackish- 

 marine series in Headon Hill. Messrs. Keeping and Tawney give 

 details of both localities. We have also Ur. Wright's section of 

 Colwell Bay, and Prof. Blake's lithological notes on both localities. 

 There is practical agreement as to the facts of the succession in all 

 published observations. Both series begin above with a thickness 

 of 12 to 15 ft. of clays with Potamomya gregaria and Paludina lenta, 

 which are beautifully preserved. Below the Potamomya clays in 

 Colwell Bay we have a band of ferruginous sand rock and under- 

 lying sands, not exceeding 3 ft., then a band of limestone " 1 ft. 6in.," 

 overlying 12 ft. of clays, which again rest on some 15 ft. of sands. 

 Below the Potamomya clays in Headon Hill we have the Great 

 Limestone 20 to 30 ft. thick, usually resting on sands of variable 

 thickness, 6 to 12 ft., though separated in places by a foot of green 

 clay. In both we have an aggregate of nearly 50 ft. of strata — a 

 great thickness of fresh- water clays above, and a good thickness of 

 fresh- water sands below. With the example of the Osborne Lime- 

 stone so close at hand, it seems not unnatural to conclude that the 

 Great Limestone (the singular factor in the correlation) thinned 

 out speedily to the N., the clay and sands beneath increasing 

 correspondingly. We must point out that Prof. Blake, in arguing 

 otherwise, comes to contradictory conclusions, first stating^ that 

 " the Cerithium and Oyster beds of Colwell Bay occupy the place of 

 ferruginous sands below the Headon Hill Limestone," and, finally,^ 

 that the " Headon Hill Limestone becomes the fresh-water repre- 

 sentative of part of the marine Brockenhurst " [i.e. Colwell Bay) 

 " series, and thus becomes doubly anomalous.^' The italics ai'e ours. 

 A limestone intercalation over a limited area in an estuary is a 

 recognized vera causa, but a depression in the N., which admits 

 the sea over that part of the area, while 24 ft. of fresh- water lime- 

 stone are being formed within half a mile in the shallows of the 

 S. — where the sea too had had access not long before — is almost 

 inconceivable. 



The two " Upper Headon " Series occur in localities situated at 

 a distance of about a mile and a half. But the two brackish-marine 

 series approach within half a mile. It is no longer a comparison of 

 Colwell Bay and Headon Hill, but of the northern and southern 

 boundaries of Totland Bay. For the brackish-marine of Colwell 

 Bay can without a shadow of doubt be followed by the eye continu- 

 ing into that of Warden Point and Warden Cliff, cropping out on 

 the top of the cliff just N. of Weston Chine. The question then 

 narrows to this. Does the Totland brackish-marine of Warden 

 Cliff agree in detail with that of Headon Hill just opposite so 

 closely that we must suppose them to have been continuous ? For if 

 so, then the Colwell Bay brackish-marine, continuous with that of 

 1 q/;. cit. p. 157. 2 op^ cit. p. 160. 



